This invention relates to a circuit arrangement for switching a current through a load which comprises an inductive dummy resistor, in which at least one semiconductor switch switches the current through the load periodically by means of its main current path which is switched alternately to the conductive and the non-conductive state, which has connected parallel to it a voltage-limiting element for limiting the voltage applied to it.
Such a circuit arrangement is known from the article "Class E--A New Class of High Efficiency Tuned Single-Ended Switching Power Amplifiers" by Sokal and Sokal, I.E.E.E. Journal of Solid State Circuits, Volume SC-10, No. 3, June 1975, p. 168-176. More particularly on page 172 of this article in FIG. 3 with associated text on page 172, lefthand column, a circuit arrangement is described having a switching transistor as the semiconductor switch. The emitter of the switching transistor is connected to ground potential and its collector is connected via a choke coil to the positive terminal of a direct voltage source. A first capacitance serving as a voltage-limiting element and a series arrangement of a second capacitance, a further coil and a load resistor are connected parallel to the collector-emitter path of the transistor (constituting the transistor main current path). The transistor is connected through its base connection to a driver, by means of which it is switched periodically to the conductive and to the non-conductive state. More particularly when the transistor is switched to the non-conductive state, the voltage which occurs at its collector-emitter path is kept low by the first capacitance in the time interval during which the transistor is switched from the conductive state to the non-conductive state until the current in the collector of the transistor has fully decayed to zero. The first capacitance is moreover defined so that the voltage across the collector-emitter path has again fallen to zero when the transistor is switched for the next time from the non-conductive to the conductive state. Thus, losses in the transistor are avoided and consequently the efficiency of the circuit arrangement is increased. Moreover, a direct voltage source of a higher voltage can now be used than would be possible in a circuit arrangement that did not include the first capacitance.
Depending upon the inductive load and the choke coil, the first capacitance is charged during the time interval in which the transistor is in the non-conductive state, while voltage peaks occur across the collector-emitter path. These voltage peaks have a height equal to a multiple of the voltage of the direct voltage source. The voltage peaks may lead to a voltage breakdown of the collector-emitter path even after it has been switched to the non-conductive state, as a result of which not only do losses occur again in the transistor, but this transistor can also be destroyed.